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Caves on Mars

This series of pictures shows seven proposed cave skylights. Clockwise from upper left are Dena, Chloe, Wendy, Annie, Abbey and Nikki, and Jeanne. Arrows signify direction of solar illumination (I) and north (N).

Researchers say pictures from a Mars orbiter show holes the size of football fields that may be the entrances to subterranean caverns. If the claims prove to be true, such caves would be prime targets in the search for extraterrestrial life and prime real estate for future human settlements.

The possibilities are raised in a research paper presented last week at the Lunar and Planetary Science Conference in Texas, an annual gathering of experts in fields ranging from moon exploration to asteroid detection and astrobiology. The research hasn't yet appeared in a peer-reviewed publication, but the presentation - highlighted in news reports from Nature and the BBC - has already created a buzz among the experts.

Martian caves have long been considered the best potential havens for life, since they would be sheltered from the harsh radiation hitting the surface, as well as wild temperature shifts and brutal dust storms. The researchers acknowledge in their paper that they're talking about something of more than academic interest:

"Besides general geological interest, there is a strong motivation to find and explore Martian caves to determine what advantages these structure may provide future explorers. Furthermore, Martian caves are of great interest for their biological possibilities because they may have provided habitat for past (or even current) life.

"Preserved evidence of past or present life on Mars might only be found in caves, and such a discovery would be of unparalleled biological significance. ..."

The big question: Are the things the researchers spotted on Mars really, truly caves?

The paper's authors - Glen Cushing, Timothy Titus and J. Judson Wynne of the U.S. Geological Survey, plus Phil Christensen of Arizona State University - base their claims on an analysis of pictures from the Thermal Emission Imaging System aboard NASA's Mars Odyssey orbiter. The imager, also known as THEMIS, spotted seven weird-looking black features in an area on the flanks of the Arsia Mons volcano.

That area is prone to geological phenomena known as collapse pits, in which surface material falls into a depression or subsurface void that is generally created by seismic activity. Chains of such pits have been spotted on Mars many times before. But the researchers say these seven dark features are different, because they don't appear to have sunlit walls or floors. They say the features don't look like impact craters, because there aren't any raised rims or blast patterns. And they say that the way the black spots retain heat appears to rule out the idea that they're merely surface features of a different color.

Putting all this together, the research team concludes that the seven black spots are actually "skylights" - areas where the surface has collapsed to reveal a chamber below. But these are no bathroom-size skylights: The diameters range from 330 to 825 feet (100 to 252 meters), and the chambers below are estimated to be at least 240 feet (73 meters) deep.

The "seven sisters" have been given informal names: Dena, Chloe, Wendy, Annie, Abbey, Nikki and Jeanne.

The researchers admit that they can't determine whether the skylights are simply vertical shafts or the entrances to much larger subterranean caverns. But they did use readings from THEMIS to determine that one of them, Annie, stays warmer than the shadowed areas of nearby collapse pits during the Martian afternoon - and that it retains more heat during the night. They estimate that Annie's floor is at least 425 feet (130 meters) beneath the surface.

Such a setting would thrill any armchair Mars explorer. But when I asked Christensen about the study today, he was more circumspect - in part because it will require many more observations to determine the true nature of the seven sisters.

"Any sort of new observation is fun when you're thinking about what the nature of the caverns underneath could be," he told me. "Anything novel is always interesting. It forces you to think about new possibilities."

He said the first avenue for further observations could be provided by NASA's latest Red Planet probe, Mars Reconnaissance Orbiter. The spacecraft's high-resolution camera could take a closer look at the seven sisters - including sidelong glances that might show whether the features open up into wider chambers beneath.

Ground-penetrating radar readings, sent back by instruments on Mars Reconnaissance Orbiter or the European Space Agency's Mars Express, also might determine what lies beneath, Christensen said. But it's not yet clear whether the resolution would be good enough to produce a map of the caverns. And even if such a map can be made, that wouldn't resolve all the mysteries surrounding the seven sisters.

"I can't think of any great way to explore then other than to physically go down and explore them on the ground," Christensen said. That might require a human mission - or at least advanced robotic missions with "cliff-bots" capable of rappeling down into a hole in the ground.

"Anyplace on Mars where a hole exists - where you could get, who knows, 50 meters beneath the surface - is pretty interesting," Christensen said. "These caverns could have ice in them. They could have liquid water, who knows. I think that would be a very exciting mission.

"What you would find is anybody's guess," he said. "But that's what exploration is all about."

Update for 6 p.m. ET:Penny Boston, a biologist and self-described "born-again caver" at New Mexico Tech, says the latest report meshes well with what she's been studying over the past couple of decades. "I believe that we have been seeing what these guys have reported in many, many places on Mars - and we're building a catalog," she told me.

Boston said the types of features described last week are consistent with what geologists see in lava tubes on Earth. She said the idea that such tubes are present on Mars as well "might have been an exotic concept when we first started publishing about this a decade ago," but no more.

On Earth, the walls of collapsed lava tubes can hold deposits of permanent ice or even microbial communities. "The potential, we believe, for having collapsed lava tubes on Mars that are essentially time capsules makes them hot targets for future mission opportunities," Boston said.

The most interesting caves for Boston would be the ones that have been closed off rather than the ones that have been opened up. "They can contain things, and the fact that they have been sealed away from the ravages of the Martian surface environment makes them even more attractive," she said.

But she said the open caves would provide the best conditions for human settlements - and to her mind, the idea of building glass-bubble domes on the Martian surface is hopelessly out of date. "That's mostly ignorance on the part of the people doing the thinking," she said.

You can tell from all this that she's not an entirely disinterested observer. In fact, one of the researchers behind the latest study is a collaborator of hers on other projects.

"I'm excited about their results," she said. "It's a continuation of what we've been working toward, and I'm looking forward to collaborating with them in the future. ... On to Mars!"